Due largely to environmental concerns, there is an increasing need for low-sulfur emissions from fossil fuels such as coal which contain sulfur. Heretofore, both post-combustion and pre-combustion desulfurization techniques have been available. For example, flue gas desulfurization is a well know post-combustion process. However, it is generally inconvenient, expensive and limited with respect to the amount and types of sulfur combustion products which can be removed. Flue gas treatment also ignores other economic impacts from the handling and processing of fuels containing sulfur, such as corrosion caused by the sulfur in coal to the equipment used to handle the coal. Pre-combustion processes, on the other hand, which result in low-sulfur fuels, can reduce both sulfur emissions and equipment corrosion.
The bulk of the sulfur content of a fossil fuel exists as inorganic, pyritic sulfur (i.e., a metal sulfide) or as organic sulfur (i.e., sulfur covalently bound to carbon or a hydrocarbon moiety).
Organic and pyritic sulfur each constitute between 20 and 80% of the total sulfur content of coal, depending upon the specific coal variety.
Inorganic pyritic sulfur is generally found in coal in the form of iron pyrite which is disseminated as a separate mineral phase throughout the body of the coal and may be liberated from coal by selected physical or chemical techniques. Conventional coal desulfurization processes to remove inorganic pyritic sulfur include physical methods such as gravity flotation, magnetic, or electrical separation methods. While these physical methods are convenient and economical, they are capable of removing only inorganic (pyritic) sulfur and generally result in notable energy losses from the coal.
Chemical desulfurization methods known for the treatment of coal convert inorganic pyritic sulfur to a water-soluable sulfate form to enable the removal of the inorganic sulfur compound by water extraction. (Wilson, European Patent Application 0 010 289). While chemical coal desulfurization processes, such as oxidation with ferric salts, chlorine or ozone, or reduction with a solvent-hydrogen mixture or alkali, may be effective in removing some types of organic sulfur, many types of organic sulfur are not susceptible to attach by chemical reagents. In addition, these methods generally have numerous disadvantages, such as, corrosion problems from reagents, high energy requirements, and costly reagent recovery and loss of desirable qualities of the coal.
Richardson (U.S. Pat. No. 4,256,485) suggests that coal may be treated with oxidative enzymes produced in situ by the fermentation of yeast. The oxidative enzymes produced by this live yeast system convert inorganic pyritic sulfur to inorganic sulfate for removal by water extraction. As with chemical oxidation methods, enzymatic oxidation by live yeast cells may also enable the water extraction of some types of organic sulfur compounds.
Attempts have also been made to remove inorganic and organic sulfur from coal by microbiological methods. Early interest in this field focused on microorganisms which were naturally suited for pyritic sulfur digestion, such as Thiobacillus found in mine waters and Sulfolobus found in sulfur springs, as reported in Detz et al, American Mining Congress Journal, vol. 65, p. 75 (1979); Kargi et al, Biotechnology and Bioengineering, vol. 24, pp. 2115-2121 (1982). Such bacteria are effectire in removing only inorganic pyritic sulfur and have no propensity for organic sulfur removal.
Although such processes as Wilson European Patent Application 0 010 289 and Richardson, U.S. Pat. No. 4,256,485 reduce the total sulfur content of a fossil fuel, the reduction generally corresponds only to the amount of inorganic pyritic sulfur present in the fossil fuel. Such processes are not effective for substantially reducing the organic sulfur content of the fossil fuels. Consequently, the treated fossil fuel often retains an objectionable high sulfur content.
Theoretically, organic sulfur cannot be removed from coal unless the chemical bonds holding the sulfur are broken or the organic sulfur compound is extracted (Encyclopedia of Chemical Technology, Vol. 6, John Wiley & Sons, pp. 306-324, 1979). Because organic sulfur is an integral part of the chemical structure of the coal, it has not been possible to remove organic sulfur from coal without severely disrupting the chemical bonding which occurs within the structure of the coal. Those processes which have been successful in removing organic sulfur from coal require extreme process conditions, e.g. pressure and temperature, are expensive, and require the input of large quantities of energy.
More recently, efforts have focused on the adaptation of microorganisms for organic sulfur removal. Such attempts are reported, for example, in Isbister et al, "Microbial Desulfurization of Coal", in Attia (ed), Processing and Utilization of High Sulfur Coal, p. 627 (1985); and Robinson and Finnerty, "Microbial Desulfurization of Fossil Fuels" (University of Georgia) and Stevens, U.S. Pat. No. 4,659,670. There are, however, numerous obstacles which must be overcome before such microbial techniques become practical. For example, optimal growth conditions in a large scale process are difficult and expensive to maintain, typically requiring expensive growth factors and excessive nutrients or additives. Such additives themselves can be a potential environmental concern and possibly as difficult to remove economically as the sulfur. The growth of the microorganisms can also produce toxic by-products or compounds which may result in mortality or render the microorganisms incapable of catabolizing sulfur. In addition, such fermentation processes are usually plagued with problems such as culture stability, mutation or contamination, reactor upsets, substrate variation, and the like.
There remains an unfilled need for an economical and efficient method for desulfurizing coal and other fossil fuels which method significantly removes both organic and inorganic types of sulfur.